Ultra-violet radiation-desensitizable thermographic copy-sheet and method

ABSTRACT

997, 362. Thermographic copying material and process. MINNESOTA MINING &amp; MANUFACTURING CO. Jan. 3,1962 [Jan. 3,1961], No. 10700/65. Divided out of 997,361. Heading G2C. A copy sheet or copysheet combination, which forms a visible image when heated but can be desensitized by exposure to ultra-violet radiation, contains (1) a normally solid organic salt of nickel, cobalt or copper and (2) N, N&lt;SP&gt;1&lt;/SP&gt;-bis-furfuryl-dithiooxamide or N, N&lt;SP&gt;1&lt;/SP&gt;-bis-carboxyamyl-dithiooxamide. Copies may be made by placing an original in contact with the copy sheet or combination, irradiating with infra-red while in contact, and subsequently desensitizing the copy by exposure to ultraviolet, or by exposing a copy sheet to ultra-violet through a transparent original bearing opaque markings, and subsequently heating to develop colour in the areas not so desensitized. The two reagents may be disposed in a single layer, in separate layers on a single carrier, or in separate layers on separate carriers. The reagent (1) may be nickel stearate or behenate. The reagents may be dispersed in a thin film of a resinous or polymeric film-forming binder in the form of a self-sustaining film, or as a coating on a backing of film, paper or the like. The Specification includes a list of suitable binders. The sheets may also include pigments, fillers, plasticizers, dyes, waxes and resins.

United States Patent O 3,094,619 ULTRA-VIOLET RADIATION-DESENSITIZA- BLE THERMOGRAPHIC COPY-SHEET AND METHOD Edwin A. Grant, In, St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware N Drawing. Filed Jan. 3, 1961, Ser. No. 79,990 7 Claims. (Cl. 250-65) This invention relates to heat-sensitive copy-sheets and methods of making and using the same.

One well-known and commercially important method for copying typewritten office correspondence and other graphic originals having differentially radiation-absorptive image and background areas is known as thermographic reproduction. It involves the brief irradiation of the original, while in heat-conductive contact with a heatsensitive copy-sheet, with high-intensity radiation. Preferential absorption of the radiation in the absorptive image areas then results in localized heating of the copy-sheet, producing a visible direct copy of the original. The background areas of the copy-sheet remain heat-sensitive.

Stably heat-sensitive copy-sheets having a heat-sensitive layer containing a normally solid organic acid salt of a noble metal and a cyclic organic reducing agent for the noble-metal ions, maintained in physically distinct and chemically inter-reactive relationship, have been found effective in thermographic copying procedures as just described. Silver salts such as silver behenate, and reducing agents such as methyl gallate, dihydroxybenzoic acid, or 4 methoxy 1 hydroxy dihydronaphthalene have been found to provide particularly desirable copy-sheets. The coated sheet materials are substantially free of color. When locally briefly heated, as in the thermographic copying process or by brief contact with a metal test bar at a conversion temperature within the range of about 90 C. to about 150 0, they are permanently visibly darkened and the darkened areas are then found to be differentially radiation-absorptive. However the background areas remain visibly heat-sensitive.

The present invention makes possible the preparation, on heat-sensitive copy-sheets containing silver salt and organic reducing agent as the image-forming reactants, of permanent copies having sharply defined colored image areas on visible heat-insensitive substantially colorless backgrounds.

It has now been found that heat-sensitive copy-sheets prepared with silver behenate or equivalent noble metal salt and certain substituted alpha-naphthols as the interreactant components of the visibly heat-sensitive layer may be rendered incapable of undergoing the thermographic image-forming reaction by moderate exposure to nearultraviolet radiation. The invention therefore makes possible the preparation of permanent, heat-stable, high image density reproductions of graphic originals by procedures involving only irradiation and without any necessity of treatment with solutions, fumes, vapors, powders or other adjuvants.

Typically, the composite of copy-sheet and original is first briefly irradiated with intense radiation rich in infrared and in accordance with known thermographic copying procedures. The thus imaged copy-sheet is next exposed to radiation in the near-ultraviolt wavelength range, such for example as is obtained from a mercury vapor arc. The heat-sensitivity of the background areas is destroyed without any visible change in appearance of either the background or the image areas.

Copies may alternatively be prepared by first exposing the copy-sheet to the near-ultraviolet through a transparency, for example a ribbon or carbon copy of a typewritice ten document on thin semi-transparent paper, and then heating the entire copy-sheet to conversion temperature, for example on a heated roller or platen or in an oven.

The organic reducing agents which have been found to be useful in the practice of the invention comprise alphanaphthols having a preponderance of electron donor groups attached directly to the hydroxy-substituted aromatic ring. Alkyl, aryl, alkoxy, .aryloxy, hydroxy and amino groups are particularly effective. Illustrative compounds include: 4-methoxy-1-naphthol; 4-ethoxy-l-naphthol; B-hydroxy-l-naphthol; 4-amino-1-naphthol; 2-methyl-4-methoxy-l-naphthol; 4,4-dimethoxy-l,l-dihydroxy- 2,2-binaphthyl; 1,1'-dihydroxy-2,2'-binaphthyl; and the bis-dihydroxynaphthalene dimethylmethane condensation product of 1,5-dihydroxynaphthalene and acetone. A condensation product of two mols of 1,5-dihydroxynaphthalene and one mol of adipoyl chloride has been found useful. The compound 2-phenyl-4-methoxy-l-naphthol is also a useful reducing agent and the copy-sheet produced with this substance and silver behenate as the interreactants is made insensitive to visible change on heating when moderately irradiated with near-ultraviolet radiation.

Other radicals may also be present in these alpha-naphthol organic reducing agents but the electron donor groups must preponderate. For example, in the compounds 2- acetyl-4-methoxyl-naphthol and 2-behenoyl-4-methoxy- 1- naphthol, the alkoxy substituent has the greater effect; thus when employed in the same manner these compounds produce operable copy-sheets which, however, requires significantly higher conversion temperatures and significantly longer exposure to near-ultraviolet to achieve the desired result.

Of the several compounds named hereinbefore, 4-methoxy-l-naphthol, and compounds such as 2-methyl-4-methoxy-l-naphthol which contain additional electron donating substituent groups, provide heat-sensitive copy-sheets having the most effective combination of reactivity at readily available conversion temperatures and greatest susceptibility to the effects of moderate ultraviolet radiation; and these compounds are therefore greatly preferred for use in the copy-sheet of the present invention.

The reducible metal ion is preferably provided in the form of silver behenate or silver stearate, but other normally solid organic acid salts of noble metals are also useful. The water-insoluble salts of the higher fatty acids are preferred for their moisture-resistance properties.

The substituted alpha-naphthol and noble metal salt may be intimately dispersed in a thin film of a resinous or polymeric film-forming binder, either in self-sustaining film form or as a coating on a paper, film or other paperlike backing, providing a unitary heat-sensitive copy-sheet in a single coating and drying operation. The two reactants may separately be dispersed or dissolved together with appropriate binders, and applied to a paper-like backing in separate coatings'with intermediate drying, or to separate backings. In the latter case the two sheets, when held in face-to-face contact, permit interfacial reaction at conversion temperature. The relative amounts of the two reactants are not critical, particularly in structures wherein the image-forming reaction occurs at an interface, the only requirement being that suflicient of each reactant be present to insure a readily visible change on reaction. It is desirable to employ a minimum amount of the alphanaphthol and to maintain this material in position for maximum exposure to the actinic radiation.

Typical resinous or polymeric film-forming binders include polystyrene, poly-t-butyl methacrylate, polyvinyl acetate, ethyl cellulose, polyisohutylene, and copolymers of styrene and isobutylene. The reactants are preferably supplied, together with small amounts of binder, as a coating on a paper-like support as previously noted, but may alternatively be retained in a fibrous sheet structure in the substantial absence of binder, or in a thin self-sustaining binder film in the absence of the paper-like carrier.

Various additives are useful in providing copy-sheets of improved appearance, handling characteristics, or other properties. Pigments, fillers, plasticizers, dyes, waxes, re sins and other additives may be incorporated. The addition of various toning agents is particularly desirable with compositions employing silver soaps, for the purpose of improving the color and density of-the resulting image.

The following specific examples will further illustrate but not limit the invention. Proportions are given in parts by weight unless otherwise specified.

Example 1 A mixture of equal mol percent of silver behenate and behenic acid is prepared by reacting together one mol of silver nitrate and two mols of sodium behenate, made from commercial behenic acid, in aqueous medium and under slightly acidic conditions. The resulting waterinsoluble precipitate is recovered on a filter and is washed and dried to produce a fine powder. The powder fuses at about 135 C. and melts to a liquid at about 175 C. Silver 'behenate and behenic acid, prepared separately from the same sodium behenate, melt at about 220 C. and about 70 C. respectively.

A dispersion of 1.35 grams of the silver soap mixture in a solution of 0.9 g. of Dow 700 polystyrene resin in 6.75 g. of Z-butanone as the volatile liquid vehicle is prepared by prolonged ball milling. Just before coating there is added 0.2 g. of 4-methoxy-1-naphthol and an additional 11.0 g. of Z-butanone which has been saturated with a small amount of phthalazinone (more accurately identified as l-(2H)-phthalazinone). The mixture is coated on a white paper carrier web, using a coating knife set at an orifice of 2 mils (0.002 inch), and dried at room temperature. The resulting sheet is white in color, changing to an intense blue-black on brief contact with a metal test bar at 120 C.

The sheet is placed in contact with a printed original which is then briefly intensely irradiated with radiation from an incandescent filament in accordance with wellknown thermogr-aphic copying processes. Image areas corresponding to the inked areas of the original are converted to blue-black; background areas remain white and still heat-sensitive.

The copy is then further subsjected to prolonged irradiation with an S-Wat-t black light blub (G.E. F 8T5/ BL), whereupon the background areas are found to be nolonger visibly heat-sensitive but to remain while on contact with the heated metal test bar or under the thermographic copying process. The previously darkened image areas remain unchanged.

The dark image areas are absorptive of infra-red radiation, whereas the non-darkened background areas both before and after desensitization do not absorb any significant amount of infra-red; hence the stabilized copies produced on the heat-sensitive copy-sheets of this invention are useful as graphic originals in the preparation of further copies by thermographic reproduction processes.

Example 2 Silver behenate is prepared as in Example 1 but from equimolar quantities of silver nitrate and sodium be- :henate. Ten grams of the' salt is dispersed by ball milling in a solution of g. of Lucite 44 tertiary butylmethacrylate polymer in 90 g. of commercial heptane, and the dispersion is coated on map overlay tracing paper at an orifice of 3 mils. Over the dried coating is next applied a solution of 0.05 g. of 4-methoxy-l-naphthol, a trace amount of phthalazinone, and 1 g. of the Lucite binder resin in 9 g. of acetone, and the sheet is again dried. The sheet produces effective reproductions of printed originals by thermographic copying processes. Exposure of the copy thus obtained to radiation from a BH6 high pressure mercury arc lamp for 2 minutes at a distance of 6 inches renders the background areas no longer visibly heat-sensitive.

Example 3 Thin paper coated with a dispersion of silver behenate in a solution of Lucite 44 in heptane is dried and is then further coated with a solution of 0.5 gram of 4- rnethoxy-l-naphthol and 0.25 gram of phthalazinone in 18 m1. of methanol. The solution is applied in minimum amount by means of squeeze rolls, and the sheet is dried, providing a heat-sensitive copy-sheet which turns black when heated to conversion temperature but is made insensitive to such heat change by irradiation to near-ultraviolet for one minute under the conditions shown in Example 2.

Example 4 Paper is coated with a mixture of 10 g. of silver behenate, 5 g. of phthalazinone toner, 3 g. of Parapol polystyrene binder and 82 g. of heptane, applied at a coating orifice of 3 mils and dried. Each of a series of solutions of 0.01 g. of reducing agent in 6 ml. of a solution of 7 parts of ethyl cellulose binder in a mixture of 63 parts of methanol and 7 parts of water is then overcoated on a separate portion of the coated paper, again at an orifice of 3 mils. The resulting dried sheets are irradiated, through a mask or stencil, with UV light from the B-H6 lamp at 8 inches for 3 minutes, and are then placed in contact with black paper and briefly intensely irradiated with light high in infra-red as obtained in thermographic copying machines. The black paper absorbs the radiation to produce a uniform heating efiect. The following compounds serve effectively as photosensitive reducing com-pounds, only the masked areas becoming effectively darkened in the procedure described.

4-methoxy-1-naphthol 2-methyl-4-methoxy-l-naphthol 4-ethoxy-1-naphthol 4,4'-dimethoxy-l,l-dihydroxy-2,2'-binaphthyl 1,4-dihydroxy naphthalene 1,3-dihydroxy naphthalene Condensation product of 1,5-dihydroxy naphthalene and acetone 4-amino-1-naphthol Additional coatings may be applied where desired, for example as. color or opacifier coatings, protective surface coatings, and for other purposes. In one specific structure the carrier web employed is transparent, the heatsensitive layer is formed of a single coating containing all of the required reactant materials in a transparent binder, and a further coating of heavily pigmented binder is applied over the heat-sensitive coating, serving both as a protective layer and to provide eflective contrast against the dark image areas formed in the heat-sensitive layer during thermographic reproduction. In such a construction the carrier web most obviously be transparent. Likewise, in constructions in which the visibly heatsensitive layer is formed of two coatings, the use of a transparent backing permits the application of the photosensitive reducing agent as a first coating, 'followed by the silver salt and other components, and the opaque surface coating if desired; desensitization of such a sheet is then carried out with the transparent carrier web toward the source of ultraviolet radiation.

What is claimed is as follows:

1. A heat-sensitive copy-sheet capable of undergoing permanent visible change on being momentarily heated to a conversion temperature within the range of about C. to about C. and of being made insensitive to such heating on moderate exposure to near-ultraviolet radiation, said copy-sheet including a visibly heat-sensitive layer containing, in physically distinct and chemically inter-reactive relationship, chemically inter-reactant components consisting essentially of (1) a normally solid organic acid salt of a noble metal and, as an organic reducing agent for the noble metal ion. (2) an alphanaphthol having attached directly to the hydroxy-substituted aromatic ring a preponderance of electron donor groups selected horn the class consisting of alkyl, aryl, alkoxy, aryloxy, hydroxy and amino groups.

2. A heat-sensitive copy-sheet capable of undergoing permanent visible change on being momentarily heated to a conversion temperature within the range of about 90 C. to about 150 C. and of being of made insensitive to such heating on moderate exposure to nearultraviolet radiation, said copy-sheet including a visibly heat-sensitive layer comprising a water-insoluble filmforming binder and, in physically distinct and chemically inter-reactive relationship, chemically inter-reactant components consisting essentially of (l) a normally solid organic acid salt of silver and, as an organic reducing agent for the silver ion, (2) an alpha naphthol having attached directly to the hydroxy-substituted aromatic ring a preponderance of electron donor groups selected from the class consisting of alkyl, aryl, alkoxy, aryloxy, hydroxy and amino groups.

3. A unitary heat-sensitive copy-sheet capable of undergoing permanent visible change on being momentarily heated to a conversion temperature within the range of about 90 C. to about 150 C. and of being made insensitive to such heating on moderate exposure to near-ultravoliet radiation, said copy-sheet comprising a thin papelike backing and a visibly heat-sensitive coating thereon containing a film-forming binder and, in physically distinct and chemically inter-reactive relationship, silver behenate and 4-methoxy-1-naphthol.

4. Method of copying comprising: placing in surface contact and in position for irradiation a graphic original havng differentially infra-red-absorptive image and background areas and a heat-sensitive copy-sheet having a visibly heat-sensitive layer containing, in physically distinct and chemically inter-reactive relationship, chemically inter-reactive components consisting essentially of a normally solid silver salt of an organic acid and an alphanaphthol having directly attached to the hydroxy-substituted aromatic ring a preponderance of electron donor groups selected from the class consisting of alkyl, aryl, alkoxy, aryloxy, hydroxy and amino groups; momentarily irradiating said graphic original with intense radiation rich in infra-red for promoting visible reaction in said layer at selectively heated image-defining areas; and then irradiating said layer with radiation in the near-ultraviolet wavelength range for rendering the background areas visibly insensitive to subsequent heating.

5. Method of copying comprising: subjecting a heatsensitive copy-sheet having a visible heat-sensitive layer, containing a normally solid silver salt of an organic acid and an alpha-naphthol having directly attached to the hydroxy-substituted aromatic ring a preponderance of electron donor groups selected from the class consisting of alkyl, aryl, alkoxy, aryloxy, hydroxy and amino groups, said salt and said naphthol being in physically distinct and chemically interreactive relationship, to moderate exposure to near-ultraviolet radiation in localized background-defining areas and suflicient to render said layer insensitive to visible change on heating at said areas; and then heating at least the image-defining areas of said layer to provide a visible image.

6. A radiation-desensitizable intermediate sheet product suitable for use in conjunction with a receptor sheet containing a silver soap in the reproduction of a graphic original having visibly distinct image and background areas by methods involving actinie-radiation-induced desensitization of said intermediate sheet at background areas and heat-induced reaction between inter-reactive components of said intermediate sheet and a said receptor sheet at image areas, said sheet product consisting essentially of a paper-like backing and a layer of an alphanaphthol having directly attached to the hydroxy-substituted aromatic ring a preponderance of electron donor groups selected from the class consisting of alkyl, aryl, alkoxy, aryloxy, hydroxy and amino groups.

7. A radiation-desensitizable intermediate sheet product as defined in claim 6 in which the alpha-naphthol is 4- methoxy-l-naphthol.

References Cited in the file of this patent UNITED STATES PATENTS 2,482,815 Urback Sept. 27, 1949 2,877,167 Chalkley Mar. 10, 1959 2,910,377 Owen Oct. 27, 1959 2,936,276 Chalkley May 10, 1960 2,967,785 Allen et a1. Jan. 10, 1961 

1. A HEAT-SENSITIVE COPY-SHEET CAPABLE OF UNDERGOING PERMANENT VISIBLE CHANGE ON BEING MOMENTARILY HEATED TO A CONVERSION TEMPERATURE WITHIN THE RANGE OF ABOUT 90* C. TO ABOUT 150* C. AND OF BEING MADE INSENSITIVE TO SUCH HEATING ON MODERATE EXPOSURE TO NEAR-ULTRAVIOLET RADIATION, SAID COPY-SHEET INCLUDING A VISIBLY HEAT-SENSITIVE LAYER CONTAINING, IN PHYSICALLY DISTINCT AND CHEMICALLY INTER-REACTIVE RELATIONSHIP, CHEMICALLY INTER-REACTANT COMPONENTS CONSISTING ESSENTIALLY OF (1) A NORMALLY SOLID ORGANIC ACID SALT OF A NOBLE METAL AND, AS AN ORGANIC REDUCING AGENT FOR THE NOBLE METAL ION. (2) AN ALPHA NAPHTHOL HAVING ATTACHED DIRECTLY TO THE HYDROXY-SUBSTITUTED AROMATIC RING A PREPONDERANCE OF ELECTRON DONOR GROUPS SELECTED FROM THE CLASS CONSISTING OF ALKYL, ARYL, ALKOXY, ARYLOXY, HYDROXY AND AMINO GROUPS.
 4. METHOD OF COPYING COMPRISING: PLACING IN SURFACE CONTACT AND IN POSITION FOR IRRADIATION A GRAPHIC ORGINAL HAVING DIFFERENTIALLY INGRA-RED-AB ORPTIVE IMAGE AND BACKGROUND AREAS AND A HEAT-SENSITIVE COPY-SHEET HAVING A VISIBLY HEAT-SENSITIVE LAYER CONTAINING, IN PHYSICALLY DISTINCT AND CHEMICALLY INTER-REACTIVE RELATIONSHIP, CHEMICALLY INTER-REACTIVE COMPONENTS CONSISTING ESSENTIALLY OF A NORMALLY SOLID SILVER SALT OF AN ORGANIC ACID AND AN ALPHANAPHTHOL HAVING DIRECTLY ATTACHED TO THE HYDROXY-SUBSTITUTED AROMATIC RING A PREPONDERANCE OF ELECTRON DONOE GROUPS SELECTED FROM THE CLASS CONSISTING OF ALKYL, ARYL, ALKOXY, ARYLOXY, HYDROXY AND AMINO GROUPS; MOMENTARILY IRRADIATING SAID GRAPHIC ORIGINAL WITH INTENSE RADIATION RICH IN INFRA-RED FOR PROMOTING VISIBLE REACTION IN SAID LAYER AT SELECTIVELY HEATED IMAGE-DEFINING AREAS; AND THEN IRRADIATING SAID LAYER WITH RADIATION IN THE NEAR-ULTRAVIOLET WAVELENGTH RANGE FOR RENDERING THE BACKGROUND AREAS VISIBLY INSENSITIVE TO SUBSEQUENT HEATING. 